Brushless d.c motors have a number of advantages when used to drive centrifugal fans in terms of safety, flexibility of performance and low power consumption as compared to a.c motors. They allow lower speed operation which reduces aerodynamic noise and they have the flexibility to match the air flow to the system cooling requirements by changing the fan speed at will.
However, brushless d.c fans suffer from three major disadvantages when used in low noise level applications; namely discrete tones generated at the blade passing frequencies, bearing noise and tones produced by motor vibrations at harmonics of the DC motor torque pulsation frequency. The latter is more predominant the quieter the broad band aerodynamic noise, and can be a limiting factor in minimising installed fan noise. In addition the rotational speed of a DC fan can vary with back pressure depending on the motor design and this can cause a wide variation in noise level as the motor pulsation frequency may coincide with the motor and fan structural vibration modes.
Motor noise on any application has a higher priority if a low speed impeller design is used when the aerodynamic noise can be reduced to a minimum. The advantages of BLDC motors, in terms of noise, can be problematical. Firstly, since the motor is synchronous at all speeds is capable of producing high accelerating torque during start up. This results in high currents which in conjunction with the large di/dt associated with a square waveform can generate acoustical noise at harmonics of the pulsation frequency. The wave form can only be smoothed by use of additional components leading to increased cost and reduced reliability.
The high starting current can be limited by an increase in magnetic reluctance but this results in increased motor speed with load. A phenomenon of centrifugal fans is that the motor load varies inversely with changing back pressure, increasing speed and aerodynamic noise. Although such speed variations may be a disadvantage in terms of noise, significant benefits are offered to a designer who requires constant airflow at a varying system static pressure loss.
The motor design is therefore a compromise between conflicting requirements of cost, air flow performance and acoustics, the latter being determined by the interaction between the motor and the fan structure.
This aspect of fan design is discussed further in a paper entitled "Control of fan acoustic noise through motor design" (Hsien-Sheng Pei, Internoise 88). Other aspects of fan noise are discussed in a paper entitled "Fan noise--Generation Mechanisms and Control Methods" (W. Neise, Internoise 88).
Various solutions to the problems of noise control in centrifugal fans have been proposed in the prior art. In GB patent No. 1421207, for a "Rotary Blower Arrangement," a unitary assembly of a drum-rotor impeller and drive motor is mounted on the blower casing by means of an angle bar. The angle bar is connected to a scroll, forming part of the housing, by elastic material vibration dampers so that excitation of the blower casing is reduced.
In GB patent application 2055969A for a "Low Noise Centrifugal Blower", a curved "cut-off" portion at the termination of the involute wall portion at the blower outlet is replaced by a linear extension wall portion to eliminate a discrete operating whistle. There is, however, no overall reduction in noise levels and the whistle eliminated is purely aerodynamic in origin.
Finally, to complete the review of the prior art, an a.c motor fan scroll which is cut back from the conventional fully developed form has been employed in the IBM 9335 Model BO1 disk storage unit. In the 9335, dual impellers and scrolls are mounted to either side of a central a.c motor which is itself supported on a separate fan housing. The fan scrolls were cut back in the 9335 purely so that they could be fitted into the available space and attached to a common plate. An airstream splitting arrangement is attached to the opposite side of the plate to split and direct the emergent airstream to different parts of the disk storage unit. A.c motor fans have much simpler resonance problems than d.c motor fans and these are cured relatively simply in the 9335 by resiliently mounting the motor on the fan housing to isolate the housing from mains frequency vibration.